I wish to recreate the below SQL, what's the cleanest way to do this in Tableau? Do I have to make two new fields with the newly manipulated name, if so how do I write that one equals the other (this is one datasource so I don't think I blend) ?

When I try to use your method I get all False, where in fact many of the numbers match (sorry not in screenshot) - the fields I'm matching on use text editing commands such as RIGHT(), LEFT() - but I'm guessing that shouldn't make a diefference and it should only be compairiing the results ? Which if true I don't understand why it's not working ?

This is my Boolean Calculation, I've tried chenging fileds from Dimensions to Measures also :-(

As Dan Cory said, it seems like you're trying to perform the join operation on basis of calculated field. The sql query first performs cross join and then filtering the rows on basis of where clause which basically works as inner join.

Thank you so much your help is much appreciated, this has been bugging me for ongoing 2 weeks. Never knew you could perform those type of Joins within Tableau, this opens up other possibility's for me.

I'll give you the correct answer as you took time screen dump in stages the solution which was what I needed.